Holographic data memory

ABSTRACT

A holographic data storage medium ( 1 ) has a polymer film which is set up as a storage layer ( 2 ) whose refractive index can be changed locally by heating. The polymer film is set up for the storage of optical phase information via the local optical path length of the polymer film, which is illuminated in transmission when the information is read out. To the polymer film, there can be assigned an absorber dye ( 8 ), in order to at least partly absorb a write beam to input information and to give up the heat produced in the process at least partly locally to the polymer film ( 2 ). Preferably, a reflective layer ( 6 ) is arranged behind the polymer film ( 2 ).

[0001] The invention relates to a holographic data storage medium which can be used, for example, for storing image data such as photos, logos, text, and so on but also for the storage of other data.

[0002] In a hologram, optical phase information about an object is contained distributed over the surface of the hologram, from which an image of the object can be reconstructed when it is irradiated with light, in particular coherent light from a laser. Holograms are used in industry in many ways, for example in the form of largely counterfeit-proof identifications. Identifications of this type will be found, for example, on credit cards or cheque cards; as what are known as white light holograms, they show a three-dimensional image of the object represented even when lit with natural light. Photographically produced holograms and embossed holograms are widespread, in which a relief structure is embossed into the surface of a material, at which the light used to reproduce the object is scattered in accordance with the phase information stored in the hologram, so that the reconstructed image of the object is produced by interference effects.

[0003] WO 00/17864 describes a data storage medium having an optical information carrier which contains a polymer film set up as a storage layer. The polymer film consists, for example, of biaxially oriented polypropylene. In the previously disclosed data storage medium, the polymer film is wound spirally in a plurality of layers onto a core, there being an adhesive layer in each case between adjacent layers. Information can be written into the data storage medium by the polymer film being heated locally with the aid of a write beam from a data drive, as a result of which the refractive index of the polymer film and the reflective capacity at the interface of the polymer film change locally. This can be registered with the aid of a read beam in the data drive, since the read beam is reflected locally more or less intently in the interface of the polymer film, depending on the information written in. By focusing the write beam or read beam, information can specifically be written into and read from a preselected layer of the information carrier.

[0004] It is an object of the invention to provide a holographic data storage medium which is cost-effective and has wide possible applications.

[0005] This object is achieved by a holographic data storage medium having the features of claim 1 and the use of a data storage medium according to claim 11. A method of putting information into such a data storage medium is specified in claim 13, a method of reading information from such a data storage medium in claim 17. Advantageous refinements of the invention are listed in the dependent claims.

[0006] The holographic data storage medium according to the invention has a polymer film set up as a storage layer, whose refractive index can be changed locally by heating. The polymer film is set up to store optical phase information by means of the local optical path length in the polymer film, which is illuminated in transmission when reading out information. In the polymer film, it is therefore possible to store phase information locally, that is to say in a region provided for the storage of a unit of information, by the refractive index being changed by heating in this region. The local change in the refractive index effects a change in the optical path length of the radiation used when reading information out of the polymer film (the radiation illuminating the polymer film in transmission). This is because the optical path length is the product of the geometric path length and the refractive index; by means of a change in the refractive index, therefore, the local phase angle of the radiation used when reading out information may be influenced, that is to say the desired phase information may be stored. The local region for storing a unit of information (referred to as a “pit” in the following text) typically has linear dimensions (that is to say, for example, a side length or a diameter) of the order of magnitude of 0.5 μm to 1 μm, but other sizes are also possible.

[0007] The holographic data storage medium according to the invention may therefore be used as a refractive phase hologram or transmission hologram. In this case, the desired phase information is stored locally via a change in the refractive index and, in order to read out information, the polymer film is transilluminated. As opposed to this, in the data storage medium disclosed by WO 00/17864, the reflective capacity at the interface of the polymer film is changed, in order to be able to register differences in the amplitude of a reflected read beam during the read operation. The holographic data storage medium according to the invention is cost-effective and can be applied in many ways, as emerges from the following text.

[0008] In the holographic data storage medium according to the invention, the polymer film is preferably biaxially oriented, for example by being prestressed in two mutually perpendicular directions within its plane during production. In an oriented polymer film, a high energy density is stored in the film material. As a result of heating by depositing a relatively low quantity of energy per unit area, for example with the aid of a write beam, a relatively high material change (for example densification of material) by reformation can be achieved, which results in a local change in the refractive index and therefore in a change in the optical path length in the polymer film. Biaxially oriented polymer films may be produced from mass-produced plastics, so that the holographic data storage medium is cost-effective.

[0009] Suitable materials for the polymer film are, for example, polypropylene, polyester or polyvinylchloride, polymer films which have such a material preferably being biaxially oriented. High temperature stability and therefore an improved resistance to ageing and storage stability of the holographic data storage medium, and an increased security against data loss arising from ageing processes may be achieved with polymer films which have an elevated crystallite melting point. In this case, the crystallite melting point is preferably at least 170° C. Examples of such materials are polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polymethylpentene (PMP; also poly-2-methylpentene) and polyimide, a polymer film from such materials preferably also being biaxially oriented.

[0010] Preferred thicknesses of the polymer film lie in the range from 10 μm to 100 μm, preferably around or below 50 μm, but also smaller and greater thicknesses are conceivable.

[0011] In a preferred refinement of the invention, a reflective layer (for example of aluminium) is arranged behind the polymer film. The radiation used to read out information, that is to say in particular laser light, is thrown back at this reflective layer, so that the polymer film is illuminated twice in transmission when information is read out. The distance between the pits and the reflective layer (that is to say the thickness of the zone of the polymer film which adjoins the reflective layer and in which the refractive index is not changed locally) is preferably set up such that disruptive interference and superimposition effects are avoided. The configuration of the holographic data storage medium with a reflective layer behind the polymer film has the advantage that the arrangement can be fitted to a mechanical carrier or else directly to an object provided with information, since the image representing the stored information does not have to be reconstructed behind the polymer film.

[0012] To the polymer film, there can be assigned an absorber dye, which is set up to at least partly absorb a write beam serving to input information and to give up the heat produced in the process at least partly locally to the polymer film. An absorber dye of this type permits local heating of the polymer film which is sufficient to change the refractive index at a relatively low intensity of the write beam. The absorber dye is preferably arranged in an absorber layer arranged on the polymer film. It can also be admixed to the material of the polymer film; mixed forms are likewise conceivable. The absorber layer preferably has a thin layer (for example a thickness of 1 μm to 5 μm) of an optically transparent polymer (for example of polymethyl methacrylate (PMMA) or, in the case of applications for higher temperature, of polymethylpentene, polyetheretherketone (PEEK) or polyetherimide), which serves as a matrix or binder for the molecules of the absorber dye. The absorption maximum of the absorber dye should coincide with the optical wavelength of the write beam used, in order to achieve efficient absorption. For an optical wavelength of 532 nm of a write beam produced by a laser, for example dyes from the Sudan red family (diazo dyes) or eosin scarlet are suitable. For the common laser diodes with an optical wavelength of 665 nm or 680 nm, green dyes, for example from the styryl family (which are common as laser dyes), are more suitable.

[0013] In a preferred refinement of the invention, the holographic data storage medium has an adhesive layer for sticking the data storage medium to an object. The adhesive layer makes it possible to stick the data storage medium quickly and without difficulty to a desired object, for example to use the data storage medium as a machine-readable label in which information about the object is stored. Particularly suitable as an adhesive layer is a self-adhesive layer or a layer with a pressure-sensitive adhesive, which, in the delivered state of the data storage medium, is preferably provided with a protective covering that can be pulled off (for example of a film or a silicone paper).

[0014] Apart from the previously mentioned layers, the data storage medium according to the invention can also have additional layers, for example a protective layer of a transparent varnish or polymer which is arranged in front of the polymer film or the absorber layer, or a mechanical carrier which is located behind the reflective layer. An optional adhesive layer is preferably arranged behind the reflective layer or behind the mechanical carrier.

[0015] Information to be stored can be input into the holographic data storage medium according to the invention by means of a method in which phase information contained in a hologram of a storing object is calculated as a two-dimensional arrangement and a write beam from a writing device, preferably a laser lithograph, is aimed at a storage layer and/or possibly the associated absorber layer of the data storage medium and is driven in accordance with the two-dimensional arrangement in such a way that the local optical path length in the polymer film set up as a storage layer is set by a local change in the refractive index in accordance with the phase information. Since the physical processes in the scattering of light at a storing object are known, a conventional set-up for producing a hologram (in which, for example, coherent light from a laser, which is scattered by an object (storing object) is brought into interference with a coherent reference beam and the interference pattern produced in the process is recorded as a hologram) is simulated with the aid of a computer program, and the interference pattern and the phase information contained therein are calculated as a two-dimensional arrangement (two-dimensional array). The resolution of a suitable laser lithogragh is typically about 50 000 dpi (dots per inch). The refractive index in the polymer film can therefore be changed locally in regions or pits of a size of about 0.5 μm to 1 μm. The write speed and other details depend, inter alia, on the parameters of the write laser (laser power, optical wavelength) and the exposure duration.

[0016] The phase information is therefore preferably input into the storage layer in the form of pits of predefined size. In this case, the phase information can be stored in a pit in binary encoded form. This means that, in the region of a given pit, the polymer film assumes only one of two possible values for the refractive index. These values preferably differ considerably, in order that intermediate values occurring in practice for the refractive index which lie close to one or the other value can be assigned unambiguously to one or the other value, in order to store the information reliably and unambiguously.

[0017] Alternatively, the phase information can be stored in continuously encoded form in a pit, the local optical path length in the pit being selected from a predefined value range. This means that, in a given pit, the refractive index of the polymer film can assume any desired value from a predefined value range. In this case, the information may therefore be stored “in grey stages”, so that each pit is given the information content from more than one bit.

[0018] In a method of reading information out of a holographic data storage medium according to the invention, light, preferably coherent light (for example from a laser) is aimed over a large area onto a storage layer of the data storage medium, and the storage layer of the data storage medium is illuminated in transmission, the light possibly being reflected at the reflective layer (if one such is present) behind the polymer film set up as a storage layer. As a reconstruction of the information contained in the illuminated region, a holographic image is registered at a distance from the data storage medium, for example by using a CCD sensor which is connected to a data processing device.

[0019] The term “large area” is to be understood to mean an area which is considerably larger than the area of a pit. In this sense, for example, an area of 1 mm² is a large area. For the scheme according to which information is stored in a holographic data storage medium according to the invention and read out, there are many different possibilities. It is conceivable to read out from the data storage medium in one operation, by the entire area of the polymer film set up as a storage layer being illuminated in one operation. In particular in the case of larger areas, however, it is advantageous to divide up the information to be stored into a number or large number of individual regions (for example with a respective area of 1 mm²) and to read out the information only from a predefined individual area in one operation.

[0020] When information is read out, the illuminated region of the polymer film acts as a diffraction grating, the incident light being deflected in a defined manner as a result of the locally varying refractive index or optical path length. The deflected light forms a holographic image of the stored object. This image represents the reconstruction of the information encoded via the varying local optical path length (refractive index modulation).

[0021] The holographic data storage medium according to the invention can be used for different types of stored objects. For example, both the information contained in images, such as photographs, logos, texts, and so on, and machine-readable data can be stored and read out. The latter is carried out, for example, in the form of data pages, as they are known, the phase information contained in a hologram of a graphic bit pattern (which represents the data information) being input into the polymer film as explained. When the said data is read out, a holographic image of this graphic bit pattern is produced. The information contained therein can be registered, for example with the aid of an accurately adjusted CCD sensor, and processed by associated evaluation software. For the reproduction of images, in which high accuracy is not an issue, in principle even a simple matt disc, or, for example, a camera with an LCD screen is sufficient.

[0022] In the case of the holographic storage of machine-readable data, it is advantageous that the information does not have to be read out sequentially but that an entire data set can be registered in one operation, as explained. Should the surface of the storage layer be damaged, then, as opposed to a conventional data storage medium, this does not lead to a loss of data but only to a worsening of the resolution of the holographic image reconstructed when the information is read out, which is generally not a problem.

[0023] In the following text, the invention will be explained further using exemplary embodiments. In the drawings

[0024]FIG. 1 shows a schematic plan view of a detail from a holographic data storage medium according to the invention,

[0025]FIG. 2 shows a longitudinal section through the holographic data storage medium from FIG. 1 and

[0026]FIG. 3 shows a longitudinal section through the holographic data storage medium from FIG. 1, the processes during the reading of information being illustrated in a schematic way.

[0027]FIG. 1 is a schematic plan view of one embodiment of a holographic data storage medium 1 into which information is put. The data storage medium 1 has a polymer film set up as a storage layer 2 which, in the exemplary embodiment, consists of biaxially oriented polypropylene (BOPP) and has a thickness of 35 μm. The refractive index of bipolarly oriented polypropylene may be changed locally by heating, which can be used to store information, as explained further above. The storage layer 2 preferably has a thickness in the range between 10 μm and 100 μm, but other thicknesses are likewise possible. Examples of further advantageous materials for the polymer film of the storage layer 2 are listed further above.

[0028] In the data storage medium 1, information is stored in the form of pits 4. In the region of a pit 4, the storage layer 2 has a different refractive index from that in the zones between the pits 4; the term “pit” is therefore to be understood more generally than in its original meaning (“hole”). In this case, the information can be stored in a pit in binary encoded form, by the refractive index assuming only two different values (it being possible for one of the two values also to coincide with the refractive index in the storage layer 2 in the zones between the pits 4). It is also possible to store the information in a pit 4 in continuously encoded form, it being possible for the refractive index within the pit 4 to assume any desired selected value from a predefined value range. Expressed in an illustrative way, in the case of storage in binary encoded form, a pit is “black” or “white”, while in the case of storage in continuously encoded form, it can also assume all the grey values lying between.

[0029] In the exemplary embodiment, a pit 4 has a diameter of about 0.8 μm. Forms other than circular pits 4 are likewise possible, for example square or rectangular pits, but also other sizes. The typical dimension of a pit is preferably about 0.5 μm to 1.0 μm. FIG. 1 is therefore a highly enlarged illustration and merely shows a detail from the data storage medium 1.

[0030]FIG. 2 illustrates a detail from the data storage medium 1 in a schematic longitudinal section, specifically not to scale. It can be seen that a pit 4 does not extend over the complete thickness of the storage layer 2. In practice, owing to the writing method for inputting information, in which the storage layer 2 is heated in the region of a pit 4, the transition zone in the lower region of a pit 4 to the lower region of the storage layer 2 is continuous, that is to say the refractive index changes gradually in this zone and is not delimited as sharply as shown in FIG. 2.

[0031] Under (that is to say behind) the storage layer 2 there is a reflective layer 6 which, in the exemplary embodiment, consists of aluminium. The reflective layer 6 can fulfil its function even if it is substantially thinner than the storage layer 2.

[0032] Applied to the upper side of the storage layer 2 is an absorber layer 8. In the exemplary embodiment, the absorber layer 8 has the absorber dye eosin scarlet, whose molecules are embedded in a matrix of an optically transparent polymer, specifically polymethyl methacrylate (PMMA). In the exemplary embodiment, the absorber layer 8 has a thickness of 1 μm. Eosin scarlet absorbs light particularly well in the wavelength range around 532 nm; this wavelength is particularly suitable for a write beam from a laser lithograph for inputting information into the data storage medium 1. Examples of other materials of the absorber layer 8 are indicated further above. For example, green dyes, for example from the styryl family are particularly suitable for optical wavelengths of 665 nm or 680 nm, at which the laser diodes of current DVD appliances operate; such laser diodes can be modulated directly, which makes the pulse generation substantially simpler and cheaper.

[0033] The absorber layer 8 has a preferred optical density in the range from 0.2 to 1.0; however other values are likewise conceivable. The optical density is a measure of the absorption, here based on the optical wavelength of a write beam. The optical density is defined as the negative decimal logarithm of the transmission through the absorber layer, which coincides with the product of the extinction coefficient at the wavelength of the write beam used, the concentration of the absorber dye in the absorber layer 8 and the thickness of the absorber layer 8.

[0034] The absorber layer 8 makes it easier to put information into the data storage medium 1. This is because when a write beam is focused onto the region of a pit 4, it is at least partly absorbed in the absorber layer 8. The heat liberated in the process is largely transferred to the storage layer 2 and in this way effects a local change in the refractive index in the storage layer 2 in the region of the pit 4. However, it is possible to dispense entirely with the absorber dye if very short laser pulses are used.

[0035] In order to put information into the data storage medium 1, first of all phase information contained in a hologram of a stored object is calculated as a two-dimensional arrangement. This can be carried out as a simulation of a classical structure for producing a photographically recorded hologram, in which coherent light from a laser, after being scattered at the stored object, is brought into interference with a coherent reference beam, and the interference pattern produced in the process is recorded as a hologram. The two-dimensional arrangement (two-dimensional array) then contains the information which is required to drive the write beam of a laser lithograph. In the exemplary embodiment, the laser lithograph has a resolution of about 50 000 dpi (that is to say about 0.5 μm). The write beam of the laser lithograph is guided in pulsed operation (typical pulse duration of about 1 μs to 10 μs with irradiated power of about 1 mW to 10 mW in order to input a pit 4) over the upper side of the data storage medium 1, in order to put the desired information sequentially into the data storage medium 1 (or into a preselected region of the data storage medium 1). In the process, the write beam heats the absorber layer 8 in accordance with the two-dimensional array and in this way produces the pits 4 as explained above.

[0036]FIG. 3 illustrates in a schematic way how the information stored in the data storage medium 1 can be read out. For this purpose, coherent light from a laser (preferably of a wavelength which is only slightly absorbed by the absorber layer 8) is aimed at the upper side of the data storage medium 1. For reasons of clarity, only a small detail of this preferably parallel incident coherent light is illustrated in FIG. 3 and is designated by 10 (incident read beam). In practice, the coherent light is aimed at the storage layer 2 over a large area and covers a region of, for example, 1 mm². This is because the light originating from many pits 4 must be registered in order to reconstruct the stored information. The intensity of the incident read beam 10 is too weak to change the refractive index in the storage layer 2 and therefore the stored information.

[0037] The incident read beam 10 which, for practical reasons, strikes the surface of the data storage medium 1 at an angle, is reflected at the interface 12 between the storage layer 2 and the reflective layer 6, so that a reflected read beam 14 emerges from the interface 12 and, in the process, passes through the pits 4. Since the local refractive index of the storage layer 2 is different, depending on the pit 4, the local optical path length within the reflected read beam 14 varies, so that phase shifts occur. The result of this is that spherical waves 16 emerge from the data storage medium 1 in the manner of a diffraction grating, containing the stored phase information. At some distance from the data storage medium 1, a holographic image can be registered by a detector, having been caused by interference between the spherical waves 16.

[0038] The expenditure required for the detector and the further processing of the registered holographic image depend on the type of stored object, as already explained further above. For the reproduction of machine-readable data (data pages), a CCD sensor connected to a data processing device is particularly suitable, while for pure image reproduction, a simpler detector is practical, in particular if the image data are not to be processed further.

[0039] Apart from the layers which can be seen in FIG. 2, the data storage medium 1 can additionally have layers, for example a transparent protective layer above the absorber layer 8, and a carrier layer underneath the reflective layer 6. In preferred embodiments, an adhesive layer is arranged underneath this carrier layer or else directly underneath the reflective layer 6. With the aid of this adhesive layer, the data storage medium 1 can be stuck directly to an object. In this way, the data storage medium 1 can be used as a type of label which contains virtually invisible information which may be decoded only with the aid of a holographic construction for reading information.

[0040] If, for example, an absorber dye that is invisible in visible light (for example which absorbs in the infrared) or else no absorber dye is used, or if an absorber layer is washed away after information has been put into the data storage medium, the data storage medium may be configured to be largely transparent and very inconspicuous. A data storage medium of this type does not lead to any optical detriment of an object on which it is used as a label. 

1. Holographic data storage medium, having a polymer film set up as a storage layer (2), whose refractive index can be changed locally by heating and which is set up for the storage of optical phase information via the local optical path length in the polymer film (2), which is illuminated in transmission when information is read out.
 2. Holographic data storage medium according to claim 1, characterized in that the polymer film (2) is biaxially oriented.
 3. Holographic data storage medium according to claim 1 or 2, characterized in that the polymer film (2) has a material which is selected from the following group: polypropylene, polyester, polyvinyl chloride, polyethylene naphthalate, polyethylene terephthalate, polymethylpentene, polyimide.
 4. Holographic data storage medium according to one of claims 1 to 3, characterized in that a reflective layer (6) is arranged behind the polymer film (2).
 5. Holographic data storage medium according to one of claims 1 to 4, characterized in that an absorber dye (8) is assigned to the polymer film (2), which absorber dye is set up to at least partly absorb a write beam serving to input information and to deliver the heat produced in the process at least partly locally to the polymer film (2).
 6. Holographic data storage medium according to claim 5, characterized in that absorber dye is admixed with the material of the polymer film.
 7. Holographic data storage medium according to claim 5 or 6, characterized in that absorber dye is arranged in an absorber layer (8) arranged on the polymer film (2).
 8. Holographic data storage medium according to claim 7, characterized in that the absorber layer (8) has a binder.
 9. Holographic data storage medium according to one of claims 1 to 8, characterized by an adhesive layer for sticking the data storage medium to an object.
 10. Holographic data storage medium according to one of claims 1 to 9, characterized by stored information.
 11. Use of a data storage medium, which has a polymer film (2) set up as a storage layer whose refractive index can be changed locally by heating, as a holographic data storage medium, wherein optical phase information can be stored in the polymer film (2) via the local optical path length and wherein the polymer film (2) is illuminated in transmission when information is read out.
 12. Use according to claim 11, characterized in that the data storage medium has the features of the holographic data storage medium according to one of claims 2 to
 10. 13. Method of putting information into a holographic data storage medium (1) according to one of claims 1 to 10, wherein phase information contained in a hologram of a storing object is calculated as a two-dimensional array and a write beam of a writing device, preferably a laser lithograph, is aimed at a storage layer (2) and/or optionally the associated absorber layer (8) of the data storage medium (1) and is driven in accordance with the two-dimensional array in such a way that the local optical path length in the polymer film set up as a storage layer (2) is set by a local change in the refractive index in accordance with the phase information.
 14. Method according to claim 13, characterized in that the phase information is input into the storage layer (2) in the form of pits (4) of predefined size.
 15. Method according to claim 14, characterized in that the phase information is stored in a pit (4) in binary encoded form.
 16. Method according to claim 14, characterized in that the phase information is stored in a pit (4) in continuously encoded form, the local optical path length in the pit (4) being selected from a predefined value range.
 17. Method of reading information out of a holographic data storage medium (1) according to one of claims 1 to 10, wherein light, preferably coherent light (10), is aimed over a large area onto a storage layer (2) of the data storage medium (1), the storage layer (2) of the data storage medium (1) is illuminated in transmission, the light possibly being reflected at the reflective layer (6) behind the polymer film set up as a storage layer (2), and a holographic image (16) is detected at a distance from the data storage medium (1) as a reconstruction of the information contained in the illuminated region.
 18. Method according to claim 17, characterized in that the holographic image is detected by a CCD sensor connected to a data processing device. 